A simple microscope or magnifying glass (lens) produces an image of the object upon which the microscope or magnifying glass is focused. Simple magnifier lenses are bi-convex, meaning they are thicker at the center than at the periphery, and the image is perceived by the eye as if it were at a distance of 10 inches or 25 centimeters. Because the image appears to be on the same side of the lens as the object, it cannot be projected onto a screen. Such images are termed virtual images and they appear upright, not inverted. This interactive tutorial explores how a simple magnifying lens operates. The object (in this case the subject is a beetle) is being viewed with a simple bi-convex lens. Light reflected from the beetle enters the lens in straight lines as illustrated in the tutorial. This light is refracted and focused by the lens to produce a virtual image on the retina. The image of the beetle is magnified because we perceive the actual size of the object (the beetle) to be at infinity because our eyes trace the light rays back in straight lines to the virtual image.
The tutorial initializes with the Simple Magnifying Lens set to the highest magnification factor and an image of the beetle is projected onto the human eye retina. In order to operate the tutorial, use the Magnification Factor slider to move the lens closer or farther away from the beetle and see how the virtual image size changes.
When you look into a microscope, you are not looking at the specimen, you are looking at the image of the specimen. The image appears to be floating in space about 10 millimeters below the top of the observation tube (at the level of the fixed diaphragm of the eyepiece) where the eyepiece is inserted. The image you observe is not tangible; it cannot be grasped. It is a map or representation of the specimen in various colors and/or shades of gray from black to white. The expectation is that the image will be an accurate representation of the specimen; accurate as to detail, shape and color or intensity. The implications are that it may well be possible (and is) to produce highly accurate images. Conversely, it may be (and often is) all too easy to degrade an image through improper technique or poor equipment.
Contributing Authors
Christopher E. Steenerson and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.